This invention relates to a gas analyzer, and more particularly to an analyzer for detecting the concentration of a particular gas in a gas sample in accordance with the absorption spectrum of the gas sample. The invention will be described in the context of infrared (IR) gas analysis, although the principles of the invention are equally applicable to other optical analyzer.
It is well-known that any particular gas will absorb light in discrete portions of the spectrum. Thus, a gas can be analyzed by passing light through the gas and examining the spectral "signature". For example, a broadband source having an intensity distribution as generally illustrated in FIG. 1a may be passed through a sample of gas having many discrete absorption lines, with the detected radiation on the other side of the gas sample being generally as illustrated in FIG. 1b. When analyzing a gas sample to detect the presence of a particular gas, some form of filter would normally be used to limit the broadband radiation from a thermal source to the part of the spectrum, e.g., a particular part of the IR spectrum, in which that particular gas has its absorption lines. Thus, after filtering, the spectrum of FIG. 1a would appear as in FIG. 1c limited to a band around .lambda..sub.0 where the gas species of interest has its absorption lines, and the absorption spectrum in FIG. 1b would then appear as in FIG. 1d. The concentration of the particular gas can then be determined in accordance with the amount of light absorbed.
Ideally, one would detect separately, i.e., discriminate between, light in the vicinity of the absorption lines and rest of the IR light. The ratio between these two amounts of light would vary with absorption by the gas of interest. Interferents would tend to absorb both ranges of light equally, thus leaving the ratio unchanged. Naturally, this interference rejection is only approximate, and the changes in the spectral shape of the source could also influence the ratio to some extent.
Prior art NDIR (Non-Dispersive InfraRed) gas analyzers have not been able to directly implement this separate detection of the two parts of the spectrum in the manner described above. Instead, most methods involved operations designed to measure the total light passing through the sample and the light passing through the sample in the spectrum outside of the absorption lines. These two signals must then be subtracted to obtain the light in the vicinity of the absorption lines. For example, in the technique briefly illustrated in FIG. 2a, IR radiation passes through an absorption cell 10 containing the gas to be analyzed and then passed through a beam splitter 12 to a reference cell 14 containing a sample of the gas species of interest. The reference cell 14 contains enough of the species of interest that most of the light in the vicinity of the absorption lines is absorbed. A reference detector 16 will then detect the radiation passing through the reference cell and will provide an output signal proportional to the part of the light flux, exiting absorption cell 10, that is remote from the absorption lines. The light flux exiting the absorption cell 10 is also reflected by the beam splitter 12 to a signal detector 18 which will provide an output signal proportional to the total flux exiting the absorption cell 10.
In principle, for the analyzer shown in FIG. 2a, one could put enough of the gas species of interest into the absorption cell 10 that most of the light in the vicinity of the absorption lines would be absorbed, and then adjust the responsivity of one of the detectors so that their outputs were equal. The difference in the output signals from the two detectors would then be proportional to the light flux in the vicinity of the absorption lines, while the signal from the detector 16 would be porportional to the light outside of the absorption lines. In practical NDIR gas analyzers, an equivalent balancing of the detector responsivities usually is performed differently, but the above-described balance method illustrates the principle involved.
In an alternate arrangement shown in FIG. 2b, the reference cell 14' is moveable into and out of the path of the light flux between the absorption cell 10' and the detector 16'. With the reference cell 14' in the path of the light flux, the detector 16' would provide an output signal corresponding to the output signal from the detector 16 in FIG. 2a. With the empty cell 14" in the path of the light flux, the detector 16' would provide an output signal corresponding to the output signal from the signal detector 18 in FIG. 2a.
In either of the above techniques, the quantity of interest is the ratio of flux in the vicinity of the absorption lines to flux outside the absorption lines. Therefore, broadband absorption by the gas to be analyzed in absorption cell 10 will not change that ratio, nor will changes in the output of the infrared source. Similarly, when the gas in the absorption cell 10 has a line-type absorption spectrum, different from that of the species of interest, then on the average the two signals will be reduced proportionally. This last property is statistical--there is no guarantee that, for an especially ill-chosen filter passband (see FIGS. 1a-1d), a line-spectrum-absorbing interferent may not cause a change in the ratio of the two signals. However, in practice a very high degree of interference rejection is usually obtained.
Some disadvantages of the prior art NDIR gas analyzers are that, for those similar to the type illustrated in FIG. 2a, two detectors are required, and the responsivities must be kept properly balanced. The need for a beam splitter further complicates the optical layout. For the case of FIG. 2b, the moveable cells are relatively expensive to fabricate, requiring a drive motor, bearings, etc., all of which also tend to make the system less reliable.
A further type of known NDIR gas analyzer is the Luft cell. This device measures only the intensity of the light in the vicinity of the absorption lines, and thus other means of inteference rejection must be employed. Luft cell detectors are also relatively complex and expensive to manufacture.